Use of ceramics in fiber form and in composites is well known. Ceramics, especially in solid spherical form, useful as catalyst supports, catalysts, reflective materials, and abrasives, are also well known. In recent years, many ceramics have been made using the sol-gel process.
The sol-gel process is well known in the art--see for example U.S. Pat. Nos. 4,349,456, 3,709,706, and 4,290,847. A general reference on modern technology is A. Westwood et al., "High Technology Ceramics", Cutting Edge Technologies, National Academy of Engineering, National Academy Press, Washington, D.C. (1984) pages 117 to 130.
Preparation of hollow, ceramic microspheres by spray drying is taught in U.S. Pat. No. 4,421,562. A shortcoming of this technique is that the spheres suffer from holes and cracks.
U.S. Pat. No. 4,637,990 describes hollow, ceramic, porous microspheres prepared by a blowing technique. The resultant ceramic microspheres have diameters of 2000 to 4000 micrometers; smaller size bubbles are not taught.
U.S. Pat. No. 4,279,632 discloses a method and apparatus for producing concentric hollow spheres by a vibration technique on extruded materials to break up the material into individual, spherical bodies. This method is useful with low melting point material such as glass or metal which is fluid at elevated operating temperatures.
Non-vitreous ceramic metal oxide microcapsules and a process for making such microcapsules from sol-gel precursors is described in U.S. Pat. No. 4,349,456. The gel bubbles produced by this method are filled with organic dehydrating liquid which must be removed with considerable care to prevent fracture of the bubbles.
Hollow ceramic balls prepared by a combination of coating, sintering, and reduction are disclosed in U.S. Pat. No. 4,039,480; however, the process is complex, and the balls so obtained are large (e.g., 5 by 7 mesh size which is 2.79 to 3.96 millimeters).
Ceramic metal oxide microspheres prepared by impregnating hollow, organic resin microspheres with a metal compound and firing to remove adjuvants is disclosed in U.S. Pat. No. 3,792,136. The resultant hollow microspheres generally have large diameters of 50 micrometers to 10 millimeters (mm) and in one example, when the average diameter was 3 mm, the wall thickness is disclosed to be 17 micrometers.
U.S. Pat. No. 2,978,340 describes inorganic microspheres prepared from a fusion (melt or vitreous) process using a gassing agent. The product is not uniform in size, and the microspheres are not all hollow.
U.S. Pat. No. 3,975,194 teaches hollow microballoons prepared by an involved process. The resultant balloons have a non-uniform cross-section.
U.S. Pat. No. 4,772,511 discloses using vegetable oils or derivatives thereof as forming media for the preparation of zirconia based ceramic beads.